The present invention relates to color picture tubes, and more particularly a color picture tube of an in-line dot type intended for fine and high-quality display of characters with an improved purity and completeness of the picture image over the whole screen.
In the color picture tube of in-line dot type used, for example, as a video data terminal unit, as well known in the art, a fluorescent screen consisting of a plurality of phosphor dots is formed on the inner surface of a panel of a glass bulb serving as a vacuum envelope. A dot type shadow mask having circular apertures which are arrayed horizontally and vertically is spaced apart from the fluorescent screen by a predetermined distance. Mounted in the envelope is an in-line type three-electron-gun structure which opposes the shadow mask. Beams of electron emitted from the electron gun structure pass through apertures of the shadow mask and impinge upon the fluorescent screen to establish trios of electron beams.
With the in-line dot type color picture tube, when the electron beam is scanned in horizontal and vertical directions, three electron beams pass through one of the apertures in the shadow mask to form a trio of electron beams on the fluorescent screen. The arrangement of such a trio of electron beams is distorted at points on each scanning line as indicated in FIG. 1 due to the surface geometry of the panel and the deflecting magnetic field characteristics of the deflection yoke. As a result, a line connecting the beam trio is inclined by an inclination angle .phi. in respect of the horizontal line. This tendency is aggravated at four corners of the screen.
In a conventional shadow mask, however, apertures at the central portion and apertures at the corner are aligned horizontally and this horizontal arrangement is straightforward. When trios of electron beams are established at the corners through such a shadow mask, the interdistance between adjacent electron beams on the fluorescent screen becomes irregular as shown in FIG. 2 with the result that tolerance for color purity is degraded. More particularly, when taking trios of electron beams 20b.sub.1, 20g.sub.1 and 20r.sub.1 ; 20b.sub.2, 20g.sub.2 and 20r.sub.2 ; 20b.sub.3, 20g.sub.3 and 20r.sub.3 ; and 20b.sub.4, 20g.sub.4 and 20r.sub.4 for example, beams 20g.sub.1, 20r.sub.1 and 20b.sub.3 are spaced from each other at an equi-distance, thereby forming an approximate equilateral triangle and do not degrade the purity tolerance. However, the interdistance between adjacent beams 20r.sub.1 and 20b.sub.2 is extended whereas the interdistance between adjacent beams 20b.sub.2 and 20g.sub.3 is narrowed. Consequently, the interdistance between some of adjacent electron beams becomes irregular and the purity tolerance becomes considerably degraded as compared to one that will fully be capable of utilizing the dimensions of the trio of dots on the fluorescent screen and electron beams. The limitations on the purity tolerance cannot be eliminated whatever design correction lens is used when forming the phosphor dots.
Thus, it was the practice in the conventional art to place an inner shield inside the bulb to cure the effect of the earth magnetism, or to decrease the aperture diameter through the shadow mask. Alternatively, the difference in grading for the shadow mask is increased as compared with that of a shadow mask for the other types, i.e., electron gun and stripe shadow mask type and delta electron gun and dot shadow mask type.
However, if the difference in grading is increased, the completeness of the picture image throughout the screen becomes impaired, and hence, the quality of the picture image becomes degraded, thus imposing disadvantages on the graphic display. Decreased aperture diameter at the central portion for the overall completeness of the picture image will decrease utilization efficiency of the electron beams. On the other hand, the otherwise unnecessary excessive current flow will be needed in order to obtain the predetermined brightness, which deteriorates the focusing, damages the high resolution required of the high definition tube and increases the burden on the cathode electrode, thus shortening lifetime.